The invention relates to metal/air fuel cells, and particularly to an electrolyte flow equalization manifold for such fuel cells having recirculating electrolyte
Metal/air fuel cells or batteries produce electricity by the electro-chemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. The air cathode is typically a sheet-like member, having opposite surfaces respectively exposed to air and to the aqueous electrolyte of the cell. During cell operation, oxygen is reduced within the cathode while metal of the anode is oxidized, providing a usable electric current flow through external circuitry connected between the anode and cathode. The air cathode must be permeable to air but substantially impermeable to aqueous electrolyte, and must incorporate an electrically conductive element to which the external circuitry can be connected. Present-day commercial air cathodes are commonly constituted of active carbon (with or without an added dissociation-promoting catalyst) in association with a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element. A variety of anode metals have been used or proposed; among them, zinc, alloys of aluminum and alloys of magnesium are considered especially advantageous for particular applications, owing to their low cost, light weight and ability to function as anodes in metal/air fuel cells using a variety of electrolytes.
A typical aluminum/air cell comprises a body of aqueous electrolyte, a sheet-like air cathode having one surface exposed to the electrolyte and the other surface exposed to air, and an aluminum alloy anode member (e.g. a flat plate) immersed in the electrolyte in facing spaced relation to the first-mentioned cathode surface. A typical fuel cell unit or battery comprises a plurality of such cells.
Aqueous electrolytes for metal/air fuel cells consist of two basic types, namely a neutral-pH electrolyte and a highly alkaline electrolyte. The neutral-pH electrolyte usually contains halide salts and, because of its relatively low electrical conductivity and the virtual insolubility of aluminum therein, is used for relatively low power applications. The highly alkaline electrolyte usually consists of NaOH or KOH solution, and yields a higher cell voltage than the neutral electrolyte.
In alkaline electrolytes, the cell discharge reaction may be written: EQU 4Al+3O.sub.3 +6H.sub.2 O+4 KOH.fwdarw.4Al(OH)hd 4+K.sup.+ (liquid solution),
followed, after the dissolved potassium (or sodium) aluminate exceeds saturation level, by: EQU 4Al(OH).sub.4 +4K.sup.+ .fwdarw.4Al(OH).sub.3 (solid)+4KOH
In addition to the above oxygen-reducing reactions, there is also an undesirable, non-beneficial reaction of aluminum in both types of electrolyte to form hydrogen, as follows: EQU 2Al+6H.sub.2 O.fwdarw.2Al(OH).sub.3 +3H.sub.2 (gas)
Metal/air fuel cells are of particular interest as a fuel source for motorized vehicles and when they are used for this purpose they must be capable of being operated at an incline of at least 10.degree. to the horizontal.
Typically, the problem of feeding a plurality of fuel cells simultaneously has been solved by creating a fluid-tight stack through which fluid can be forced such that the inclination of the stack of cells does not affect fluid flow. Such an arrangement leads to refuelling difficulties as the stack must be opened for refuelling which breaks the fluid-tight seals. A basic goal in the refuelling of aluminum/air fuel cells is that the refuelling must not require the use of fluid-tight pressure seals.
U.S. Pat. No. 3,666,561 issued May 30, 1972 describes an electrolyte recirculating battery having a plurality of cells in which electrolyte is flowed in through a main manifold tube, then up through individual tubes and out of the fuel cells through individual discharge tubes. Short connectors are used between the manifold tube and the individual cells.
U.S. Pat. No. 4,520,080 issued May 28, 1985 shows an arrangement of small tubes between fuel cells for absorbing small shunt electrical currents. However, this does not apply to a typical metal/air fuel cell, but relates to a design with separate cathode and anode electrolytes.
U.S. Pat. No. 4,910,102 issued Mar. 20, 1990 describes an electrolyte flow manifold for a metal/air cell in which the manifold arrangement is at the top of the cells and the flow is from the top downwardly through each cell.
None of the above designs is suitable for simple refuelling by replacement of anodes.
It is the object of the present invention to develop a metal/air fuel cell capable of being operated in an inclined position while maintaining substantially equal electrolyte flow to individual cells and providing shunt circuit protection.